PHYSICAL PROPERTIES OF MAIN-BELT COMET 176P/LINEAR

Abstract

We present a physical characterization of comet 176P/LINEAR, the third discovered member of the new class of main-belt comets, which exhibit cometary activity but are dynamically indistinguishable from main-belt asteroids. Observations show the object exhibiting a fan-shaped tail for at least one month in late 2005, but then becoming inactive in early 2006. During this active period, we measure broadband colors of B − V = 0.63 ± 0.02, V − R = 0.35 ± 0.02, and R − I = 0.31 ± 0.04. Using data from when the object was observed to be inactive, we derive best-fit IAU phase function parameters of H = 15.10 ± 0.05 mag and G = 0.15 ± 0.10, and best-fit linear phase function parameters of m(1, 1, 0) = 15.35 ± 0.05 mag and β = 0.038 ± 0.005 mag deg−1. From this baseline phase function, we find that 176P exhibits a mean photometric excess of ∼30% during its active period, implying an approximate total coma dust mass of Md ∼ (7.2 ± 3.6) × 104 kg. From inactive data obtained in early 2007, we find a rotation period of Prot = 22.23 ± 0.01 hr and a peak-to-trough photometric range of Δm ∼ 0.7 mag. Phasing our photometric data from 176P's 2005 active period to this rotation period, we find that the nucleus exhibits a significantly smaller photometric range than in 2007 that cannot be accounted for by coma damping effects, and as such, are attributed by us to viewing geometry effects. A detailed analysis of these geometric effects showed that 176P is likely to be a highly elongated object with an axis ratio of 1.8 < b/a < 2.1, an orbital obliquity of ε ∼ 60°, and a solstice position at a true anomaly of νo = 20° ± 20°. Numerical modeling of 176P's dust emission found that its activity can only be reproduced by asymmetric dust emission, such as a cometary jet. We find plausible fits to our observations using models assuming ∼10 μm dust particles continuously emitted over the period during which 176P was observed to be active, and a jet direction of 180° ≲ αjet ≲ 120° and δjet ≈ −60°. We do not find good fits to our observations using models of impulsive dust emission, i.e., what would be expected if 176P's activity was an ejecta cloud resulting from an impact into non-volatile asteroid regolith. Since for a rotating body, the time-averaged direction of a non-equatorial jet is equivalent to the direction of the nearest rotation pole, we find an equivalent orbital obliquity of 50° ≲ ε ≲ 75°, consistent with the results of our light curve analysis. Furthermore, the results of both our light curve analysis and dust modeling analysis are consistent with the seasonal heating hypothesis used to explain the modulation of 176P's activity. Additional observations are highly encouraged to further characterize 176P's active behavior as the object approaches perihelion on 2011 July 1.